Accompanied with increasing global digitalization of television programs, demands of digital televisions are inevitably expanding day by day. Current digital televisions are divided into two main-stream categories, namely the European Digital Video Broadcasting (DVB) standard, and the American ATSC standard. American digital television specifications have an especially defined captioning standard, Digital Television Closed Captioning (DTVCC), for hearing-impaired individuals. Under a mute condition, apart from showing dialogues between characters, DTVCC also displays descriptive sentences on elements occurring on the screen. For example, descriptive sentences are displayed when there are background sounds on the screen to help hearing-impaired individuals to better understand the plot.
Video signals sent out by a DTV service provider or output from a DVD player reading a DVD containing closed captioning, are attached with the aforesaid DTVCC, which is displayed on a screen by way of a built-in closed captioning decoding device in an ATSC digital television. According to prior art, a closed caption decoding device is achieved using a special chip independently provided outside of a micro processing unit and a graphic engine. Beneficial from improved performance of a television controller having a micro processing unit and a graphic engine in a digital television, the decoding operation above can be completed with software executed by the television controller.
Referring to FIG. 1 showing a schematic diagram of a screen 10 showing a closed caption, CC708 of ATSC standards specifies that color index may be used to define colors of three targets, which are a foreground 11, a background 12 and a window 13. Each of the targets must be able to support at least 8 different colors or even more. Therefore, in a static random access memory (SRAM) of a television controller, a color index of prior art occupies at least 3×8=24 units to store color values of the foreground, background and window, where an actual size of the units is dependent on actual color types supported. Suppose each unit needs a length of three bits to complete a so-called palette and meet CC708 requirements, the demand of SRAM as well as hardware costs are increased. However, SRAM not only requires special processing procedures but also takes over a substantial amount of circuitry area. In addition, in prior art, when using set window attributes (SWA) for changing the color of the window 13 to another destination color, high-performance hardware is necessary for executing an enormous amount of algorithm, meaning that a higher-end micro processing unit and a large random access memory (RAM) are also needed in order to produce a picture in the destination color and having a same size as that of the window 13, while significant hardware resources are used to cover the foreground 11 and the background 12. In conclusion, the enormous amount of algorithm leads to excessive hardware demand further causing increase in power consumption and costs. Therefore, it is primary of the invention to overcome the various specified shortcomings of prior art.